The invention relates to a track locking method for an optical pickup head, in particular, to a track locking method that pre-deviates an optical lens on the actuator before locking a track based on the flicker frequency of a track error signal detected by the optical pickup head.
Conventionally, the surface and axis of an optical disc may not be uniformly distributed, and may suffer from eccentricity producing periodic radial and axial oscillations. Radial oscillations induce track flicker, and axial oscillations affect focus of the laser of the optical pickup head. A track locking mechanism is thus desirable to accurately focus on the destination track.
FIG. 1 shows a conventional optical disc device. Optical disc 11 is driven by a spindle motor 12, and a pickup head (PUH) 16 comprising an actuator 26 is borne by a sled mechanism 14. The actuator 26 is equipped with an optical lens 20 that focuses the laser. The optical lens 20 is kept in the center of a case 18 such that physical impact can be avoided. A CPU 22 coordinates operation of the described components.
The optical disc 11 comprises a plurality of tracks 24, and through PUH 16, data located at the destination track 24a is read. This operation, referred to as track seeking, is accomplished by a sled mechanism 14 moving the PUH 16 substantially above the destination track 24a, that is, a predetermined position. Thereafter, track locking is executed by an actuator 26. The actuator 26 accurately and rapidly tunes the position of the optical lens 20 with incremental axial and radial adjustments. The actuator receives a track locking signal to control the radial movement, and a focus signal to control the axial movement.
FIG. 2 shows the position of the PUH 16. Before track locking, the PUH 16 is distributed around point c, vibrating between point a and b due to disc deviation (This phenomenon, so called “run-out effect”). d1 and d2 denote the distance therebetween, substantially identical. The relative speed between the PUH 16 and the optical disc 11 is minimal at point a and b, and is maximal at point c. Conventionally, track locking is not executed until the relative speed therebetween is less than a threshold calculated from a track error signal. The flicker frequency of the track error signal, such as 13 k per second, is proportional to the relative speed, such that the track locking can be based thereon. The actuator 26 is controlled by a track locking signal to lock the optical lens 20 at point a or b.
FIGS. 3a and 3b show the movement of the optical lens 20. Initially, the optical lens 20 is in the case center 32, locked on point a of the optical disc 11, generating vibration 30, in which the vibration center 36 deviates from the case center 32. When another track seek and lock take place, because the actuator 26 responds faster than the sled mechanism 14, the optical lens 20 may complete the next track locking before the sled mechanism 14 moves to the proper position. As a result, inward deviation from the case center 32 increases, finally contacting the wall of case 18, inducing unexpected problems. Similarly, in another example, FIGS. 4a and 4b show the movement of the optical lens 20. The optical lens 20 may lock on the point b, with outward deviation also inducing the same problem. This phenomenon means that the PUH 16 is not always at case center 32 when track is locked, comprising two cases, inward deviation and outward deviation. When the optical disc device receives a sudden impact, which causes actuator 26 to lose a fix on destination track 24a, and data reading process is interrupted, such that another track seek and lock are required.